The percentage of emails opened on a mobile device has risen from just 4% in May 2009 to 20% in May 2011 while desktop client usage has declined by 11%. Webmail has shown the least change over two years, with a 4% decline.

As one might expect, among mobile devices the iPhone is leading the pack with a whopping 71.98% lead. The iPad actually appears next at 14.95% before other smartphones such as Android, which makes up 8.24% of the mobile email use.

One of the points that David Greiner, co-founder of Campaign Monitor, makes is that CSS support on mobile devices is generally far superior to support in desktop clients. If these trends continue and clients with CSS support become the norm we could be moving toward a golden age for email design, like the one we’ve seen on the web this year as HTML5 and CSS3 support has become more widespread.